In molded-case circuit breakers, various bimetallic elements or instantaneous releases in the overload release act on an unlatching point. As a result, the latter is actuated and acts on a breaker latching mechanism, by which the electrical contacts of the molded-case circuit breaker are opened.
There is the requirement that a molded-case circuit breaker must be able to interrupt a circuit in response to multiple short circuits, while it is still possible thereafter for overload trips to take place. The short circuits have the effect that the latching area of an energy store is contaminated to such an extent that undelayed trips are possible.
In such a case of contamination, overload trips are made more difficult by increased friction as a result of the contamination. In the case of conventional molded-case circuit breakers, the release force for unlatching the tripping unit after a short circuit, and consequent heavy contamination, is many times greater than before the short circuit. Contaminants collect in the unlatching region and therefore increase the friction significantly.
This has the effect that, after a short circuit, the thermal unlatching (overload tripping) required to comply with standards becomes uncertain. This problem often occurs in the case of molded-case circuit breakers with high rated currents, for example of 125 A (amperes) or 160 A. With high rated currents, the short-circuit current becomes less well confined than with low rated currents, which leads to a stronger erosion of the contact material and to a greater development of gas (pressure). Molded-case circuit breakers with low rated currents have a higher internal resistance, and consequently confine the short-circuit current better. This is not possible however with high rated currents, since the allowed heating in the customer connection region is stipulated by the standard.